This invention relates to the field of process control. More particularly, this invention relates to sensing the progress of a reaction at the surface of a substrate from outside of the reaction chamber in which the reaction takes place, and controlling the reaction based at least in part upon the sensed progress of the reaction.
Monolithic integrated circuits are typically formed in a series of steps in which layers of different materials are sequentially formed and selectively removed from portions of an underlying substrate. As the technology of integrated circuit fabrication has evolved, the desired thickness for many of the layers that are formed has decreased, thus producing integrated circuits that are generally more sensitive in nature than their predecessors. Commensurately, the thickness tolerances for these thinner layers have also decreased. This evolution has produced at least two ever more stringent conditions for integrated circuit processing. First, that layers are formed to proper thicknesses, and second that the etch processes used to selectively remove certain layers do so without either under etching or over etching the layer to be removed.
As to the first condition, devices designed to work with thinner layers tend to be more sensitive to variances in layer thickness. In other words, if a layer is specified to be a certain thickness, and a five percent variation in thickness is acceptable, the amount of actual variation in the thickness of the layer is less for a relatively thinner layer of material than it is for a relatively thicker layer of material. Thus, the processes and equipment used to form the relatively thinner layer require a greater degree of control than do those used to form the relatively thicker layer.
As to the second condition, under etching a relatively thinner layer of material may, in certain applications, have a greater negative effect on an integrated circuit than does under etching a relatively thicker layer of material. One reason for this is that, as mentioned above, an integrated circuit that is designed for use with the relatively thinner layer tends to be a more sensitive device, and is therefore more sensitive to small amounts of material that are left behind when a layer is under etched. On the other hand, an integrated circuit that is designed for use with the relatively thicker layer tends to be more tolerant under certain circumstances to small amounts of material that are left behind when a layer is under etched.
Similarly, in an over etch condition, a relatively thinner layer that underlies an etched layer is impacted to a greater degree than is a relatively thicker layer that underlies an etched layer. For example, although the same thickness of material may be removed from both the underlying relatively thinner layer and the underlying relatively thicker layer in each case, that same thickness of over etched material represents a greater percentage of the entire thickness of the relatively thinner layer, and thus tends to have a greater effect on the more sensitive integrated circuit. By contrast, that same thickness of over etched material represents a smaller percentage of the entire thickness of the relatively thicker layer, and thus tends to have a smaller effect on a less sensitive integrated circuit.
For these and other reasons there is a need for a system to more precisely monitor and control the processes by which integrated circuits are formed.
The above and other needs are met by a reaction chamber of the type used to create a reaction at a surface of a substrate disposed within the reaction chamber. A transmitter produces a transmitted beam having first characteristics, where the transmitter is disposed outside of the reaction chamber. A view port is disposed in a boundary wall of the reaction chamber, where the view port is formed of a material that is transparent at least in part to the transmitted beam. The transmitter, the view port, and the substrate are aligned such that the transmitted beam is directable to and reflected at least in part from the surface of the substrate, thereby producing a reflected beam having second characteristics.
A receiver is disposed outside of the reaction chamber, and the receiver receives the reflected beam from the surface of the substrate through the view port. The receiver also senses the second characteristics of the reflected beam and reports the second characteristics. A controller receives the second characteristics from the receiver and compares the second characteristics to the first characteristics to determine a difference between the first characteristics and the second characteristics. The difference relates to a progress of the reaction at the surface of the substrate. The controller also reports the progress of the reaction. A view port shield shields the view port from the reaction without substantially inhibiting the transmitted beam from reaching the surface of the substrate and the reflected beam from reaching the receiver. The transmitted beam is produced by the transmitter and the reflected beam is received and sensed by the receiver and the controller determines the progress of the reaction while the reaction is conducted.
In this manner the reaction is preferably continuously monitored to determine the progress of the reaction, and the controller can report the progress of the reaction so that it can be stopped when the reaction has proceeded to a desired point. By having the transmitter, receiver, and controller disposed outside of the reaction chamber, they are protected from the effects of the reaction, such as deposition or etch reactions. Further, the transparent view port is also protected from the effects of the reaction by the view port shield, which allows the transmitted beam and the reflected beam to pass through the view port shield without substantially inhibiting or altering the characteristics of the beams over time, and thus allows for continuous monitoring of the progress of the reaction.
In various preferred embodiments of an apparatus according to the present invention, the view port shield is a gas purge across an interior surface of the view port, where the gas is inert to the reaction. For example, the view port shield can be a nitrogen gas purge or an argon gas purge across an interior surface of the view port. In a most preferred embodiment, the transmitter, the receiver, and the controller are an ellipsometer. The transmitted beam can be visible light or a penetrating electromagnetic radiation. The view port is preferably formed of one of quartz and sapphire. The controller most preferably compares the progress of the reaction to a set point, and stops the reaction when the progress of the reaction is substantially equal to the set point. Further, the controller preferably displays the progress of the reaction as a numeric value, such as a thickness.
According to another aspect of the invention there is provided an improvement to a method for conducting a reaction at a surface of a substrate disposed within a reaction chamber. A transmitted beam having first characteristics is produced with a transmitter disposed outside of the reaction chamber. The transmitted beam is directed onto the surface of the substrate through a view port in a boundary wall of the reaction chamber. The view port is formed of a material that is transparent at least in part to the transmitted beam.
The transmitted beam is reflected at least in part from the surface of the substrate, thereby producing a reflected beam having second characteristics, which is received from the surface of the substrate through the view port with a receiver disposed outside of the reaction chamber. The second characteristics of the reflected beam are sensed and reported by the receiver. The second characteristics are received from the receiver with a controller. The second characteristics are compared to the first characteristics and a difference is determined between the first characteristics and the second characteristics. The difference relates to a progress of the reaction at the surface of the substrate. The progress of the reaction is reported by the controller.
The view port is shielded from the reaction with a view port shield, without substantially inhibiting the transmitted beam from reaching the surface of the substrate and the reflected beam from reaching the receiver. The steps of producing the transmitted beam, receiving and sensing the reflected beam, and determining the progress of the reaction are accomplished while the reaction is conducted.